Unlike humans, most birds have photoreceptor pigments that are sensitive to ultraviolet (UV) light, which has short wavelengths of 300–400 nm. However, for raptors such as kestrels, the purpose of this UV vision has remained elusive and puzzling. Over 15 years ago, when scientists found that the urine from Finnish voles reflected UV light, it seemed the mystery had been solved – UV vision was useful for scouting out good hunting grounds based on the abundance of UV-reflecting pee. However, 10 years later, studies suggest that the raptors' UV pigments are almost insensitive to UV and at best only detect UV light with longer wavelengths nearer 400 nm. Instead of benefiting raptor predators, UV light might be of more use to another type of prey, songbirds. These small birds have UV pigments sensitive to short wavelengths as well as UV-reflecting plumage. If raptors are truly insensitive to UV, then songbirds can use their feathers to secretly communicate with other songbirds. But are the raptors completely unaware of these UV signals or can they still use UV for hunting? With this controversy unsolved, Olle Lind, a post-doc in Almut Kelber's lab at Lund University, Sweden, decided to investigate with the help of two PhD students, Mindaugas Mitkus and Peter Olsson (p. 1819).
Lind realised that a missing key in this puzzle was that no one knew what wavelengths of light make it to the raptors' retinas: ‘Even before the light hits the retina, it's filtered through the cornea, the lens and the fluids within the eye. The whole ocular media acts as a cut-off filter and sets UV sensitivity automatically – if you can't get it [UV] to the retina, you won't be able to detect it!’ says Lind. Using eyes from euthanised raptors, the team measured how much UV was getting through the ocular media: ‘we excised the eye and then took away a piece from the back of the eye. We were then able to shine light through the eye, which wasn't stopped by the retina and at the back of the eye we had a probe instead’, explains Lind. They found that raptor eyes cut out a lot of UV light, with only UV light of higher wavelengths making it through to the retina. In one raptor, the red kite, most of the UV reaching the retina had a wavelength higher than 394 nm.
So, perhaps vole urine reflects UV light with higher wavelengths? To test this, the team collected urine from voles they had trapped in the surrounding countryside, and measured how much UV light the urine reflected – but it didn't. Lind admits that he was not entirely surprised – many other studies have characterised mammalian urine and found that it does not reflect UV. However, knowing the sensitivity of the raptor's pigments and using his newly collected measurements – UV transmittance to the retina and UV reflectance from vole urine – Lind was able to model whether vole urine can be informative in some way to raptors. He concludes: ‘there is definitely no information in the UV, they can't use UV to find vole urine – at least not with Swedish voles!’
So what about songbirds? Is enough UV light getting to the retina for the raptors to eavesdrop on communicating songbirds? Using previously collected data characterising UV reflectance of songbirds' plumage and their raptor-eye model, Lind says, ‘UV markings are not invisible to raptors; they can see them, but they might not be as conspicuous to them as they are to songbirds.’ So it would seem that raptors are neither blind nor highly sensitive to UV, and the question still remains – what are these UV-sensitive pigments doing in raptors?